that similar trypanosomes exist in the blood of the wild animals which
inhabit the region, but these have acquired by long residence in the
region immunity or adaptation to the parasite and no disease is
produced. With the gradual extension of settlement of the country and
the accompanying destruction of wild life the disease is diminishing.
Some of the inter-relations of infections are interesting. The
destruction of wild animals in South Africa which, by removing the
sources of nagana, rendered the settlement of the country possible was
due chiefly to the introduction of another infectious disease,
rinderpest, which not only destroyed the wild animals but produced
great destruction of the domestic cattle as well.

The _sleeping sickness_ has many features of interest. In the old
slavery days it was found that the negroes from the Congo region in
the course of the voyage or after they were landed sometimes were
affected with a peculiar disease. They were lethargic, took little
notice of their surroundings, slept easily and finally passed into a
condition of somnolence in which they took no food and gradually died.
There was no extension of the disease and it was attributed to extreme
homesickness and depression. A similar disease has been known for more
than one hundred years on the west coast of Africa, and attracted a
good deal of interest and curiosity on account of the peculiar
lethargy which it produced and from which it has received the name of
"sleeping sickness." Although apparently infectious in its native
haunts, it lost the power of spreading from man upon removal to
regions where it did not prevail. At first confined to a very small
region on the Niger river, it gradually extended with the development
of trade routes and the general increase of communications which trade
brings, until it prevails in the entire Congo basin, in the British
and German possessions in East Africa, and is extending north and